Small, isolated, and critically endangered: conservation genetics of the European mudminnow (Umbra krameri) in Austria

Floodplain specialist fishes have undergone disproportionate declines across Europe due to river regulation and groundwater alteration. The European mudminnow ( Umbra krameri ), now Critically Endangered in Austria, persists naturally in only two isolated populations (Fadenbach, Jesuitenbach). Field surveys were combined with microsatellite analyses to assess population structure and genetic diversity in both wild populations and in four ex situ stocks established from Fadenbach. Wild populations were strongly differentiated, indicating prolonged isolation without contemporary gene flow. Despite marked demographic fluctuations and recent habitat desiccation, both populations retained moderate levels of heterozygosity and showed no significant signal of a recent bottleneck. Whereas the estimated contemporary effective population size was critically low for the Fadenbach population, no robust estimate could be obtained for the Jesuitenbach population. All ex situ populations derived exclusively from Fadenbach and collectively retained their allelic diversity, yet showed significant divergence from one another due to founder effects and drift. These results thus identify two genetically distinct conservation units and reveal acute vulnerability masked by currently stable diversity metrics. Securing U. krameri in Austria will require hydrological restoration of suitable habitats, protection of both wild populations, and coordinated genetic management integrating in situ and ex situ conservation.